1. Field of the Invention
The present invention relates to an image forming apparatus such as an image display apparatus of flat panel type, utilizing an electron emitting device.
2. Related Background Art
In the image display apparatus including a cathode ray tube, a larger image size is requested, and a thinner structure and a lighter weight in such larger image size of the apparatus are becoming important issues. As an image display apparatus capable of realizing such thinner and lighter structure, the present applicant proposes an image display apparatus of flat panel type, utilizing a surface conduction electron emitting device. Such image display apparatus is formed as a vacuum container by seal bonding a rear plate, provided with plural electron emitting devices, and a face plate, provided with a light emitting member (for example phosphor) capable of emitting light by an electron beam irradiation and an anode electrode, across a frame member. In such image display apparatus, in order to prevent a deformation or a destruction of the plates by a pressure difference between the interior of the vacuum container constituting the display panel and the exterior, a pressure-resistant member, called a spacer, is positioned between the plates. Such spacer usually has a shape of a rectangular thin plate, of which end portions are positioned in contact with the both plates in such a manner that the surface of the spacer becomes parallel to a normal line to the plates.
In driving a display panel, a temperature fluctuation may be generated within the panel. Factors for such temperature fluctuation may be (1) an image source to be displayed, (2) an environment of use, and (3) a deficient heat conductance within a panel casing. More detailed causes of such fluctuation include generation and absorption of Joule's heat in an electron source, matrix wirings, a drive circuit etc., a heat generation by the phosphor, a temperature difference between the ambient temperature and various parts of the panel, and a radiant heat exchange for example by sunlight. Since these parameters are not constant in time and in space, the temperature distribution in the panel is generated not only along the planar direction thereof but also on an external surface of the face plate and the rear plate. Such temperature distribution, through dependent on an environment of use and an image to be displayed, causes a temperature difference of 5 to 20° C., typically about 10° C.
In case the temperature is higher in the face plate than in the rear plate in the vicinity of a spacer, an incident position of the electron beam is shifted in a direction attracted by the spacer. On the other hand, in case the temperature is lower in the face plate than in the rear plate, the position of the electron beam changes in a direction farther away from the spacer. A variation in the incident position of the electron beam, though dependent on a pixel pitch, amounts to −0.1 to 0.1 pixel pitch, in case of a pixel pitch of 0.6 mm and a temperature difference of 10° C., thereby significantly deteriorating the display quality.
For suppressing a fluctuation in the incident position of the electron beam in the vicinity of the spacer, resulting from such temperature difference between the front and rear sides of the panel, a technology is disclosed in a patent reference 1, which describes suppression of the fluctuation in the electron beam position resulting from the temperature difference between the front and rear sides of the panel, by selecting a thermal conductivity of the spacer member, a temperature dependence of a resistance thereof, a ratio of a cross section of the spacer and an area of display, and a height of the spacer within desired ranges.
Patent Reference 1: U.S. Pat. No. 5,990,614
A spacer of a high performance has to meet following requirements:    a strength and a shape capable of withstanding the atmospheric pressure;    a uniform potential distribution for designing a static potential standard;    an antistatic structure for designing a dynamic potential standard; and    a resistance designing for suppressing an electric power consumption.
It is extremely difficult to meet all these requirements with a single material. For this reason, there have been employed various methods such as forming surface irregularities, covering an insulating substrate with a high resistance film, or patterning films different in a secondary electron emission coefficient. It is also required to solve the problems such as “chemical stability”, “suppression of degassing”, “cost” and “ease of handling in manufacture”. It is therefore desired to support required design parameters, as far as possible, by parts other than the spacer.